Building test suite for known problems

Background

As we discover and fix issues with our force fields, we should prevent them from re-occurring by checking for known common problems with canary tests. OpenFF already has some tests in place, such as the HMR canary tests and a sulfonamide test:

Goal

These tests should be fleshed out. Known problematic functional groups and parameters, especially hypervalent sulfur groups and of nitrogen impropers, should be checked with molecules spanning a variety of chemistries. These only need to be run when a force field is close to release, so computational expense is not a huge concern. Moreover, simulating a ligand in water is fairly cheap.

To do